Interpretive Summary: Although many insects can tolerate low oxygen conditions, it is not known if exposure to these conditions affects long-term health. The alfalfa leaf-cutting bee is a solitary bee that constructs its nests in small hollow spaces. Because young bees develop and even spend the winter inside these spaces, they may possess built-in mechanisms that allow them to survive low oxygen conditions for long periods of time. The purpose of this study was to better understand the overwintering physiology of this bee and to answer the question of whether extended low oxygen exposure affects its long-term health. To do this, overwintering bees were incubated at one of five different oxygen levels for up to eleven months. We found that bees that were incubated in lower oxygen concentrations survived better than those incubated in normal oxygen concentrations. To study the long-term health effect of low oxygen exposure, stored bees were allowed to develop into adults, which then underwent a series of tests. These tests showed that while the size of the bees was not affected, low oxygen storage resulted in adult bees that fed less and did not live as long. Thus, while this bee can survive long periods of low oxygen exposure, the adult bees develop long-term health problems that would affect their ability to pollinate crops.

Technical Abstract:
Many insects are tolerant of hypoxic conditions, but it is unknown whether survival under hypoxia comes at any cost to long-term health. The alfalfa leaf-cutting bee, Megachile rotundata, is a solitary cavity-nesting bee. Because juvenile M. rotundata develop in brood cells inside cavities, they may possess intrinsic coping mechanisms that allow them to survive hypoxic conditions. M. rotundata overwinter as prepupae and may be exposed to hypoxic conditions for extended periods of time. This study seeks to better understand overwintering physiology in M. rotundata and answer the question of whether extended hypoxia results in sub-lethal effects. To determine the effects of hypoxia on adult physiology, overwintering prepupae were incubated at one of five different oxygen levels: 10, 13, 17, 21 and 24% O2 for eleven months. Bees reared in hypoxia had higher adult emergence rates compared to bees reared in normoxia, which were dead after eleven months. To determine whether the survival under hypoxia is associated with any costs to adult performance two groups of bees were compared: 2011 broods stored for nine months in 10% oxygen and 2012 broods stored in standard 21% oxygen. Five indicators of adult quality—emergence weight, body size, feeding activity, flight performance, and longevity —were assessed to determine whether bees that survived extended hypoxic storage possessed physiological and behavioral capacities characteristic of competent pollinators. M. rotundata prepupae exposed to extended hypoxic conditions had similar emergence weight, head width, and cross-thorax distance compared to bees reared in standard 21% oxygen. Despite no significant morphological differences between hypoxia-exposed and control bees, hypoxia-exposed bees had lower feeding rates and shorter longevity, which would compromise their ability to be effective pollinators. M. rotundata prepupae are tolerant of hypoxic condition and have higher survival rates during extended exposure to hypoxia. However, extended exposure to hypoxia, while not fatal, causes sub-lethal effects that compromise the quality of the emerging adults.